The present invention relates to a diffraction grating element used for generating three beams or the like for the tracking control in an optical pickup device for playing back a CD or the like. More particularly, the invention relates to a diffraction grating element with a mechanism for carrying out the positioning in a simple manner. Further, the invention relates to an optical pickup device with a mechanism by which the positioning of the diffraction grating element may be carried out in a simple manner.
In many optical pickup devices, which are used for recording and reproducing information to and from an optical recording medium, such as CD or DVD, a three beam method is employed in order to obtain a tracking error signal used for the tracking control.
In the 3-beam method, a laser beam emitted from a laser light source is diffracted by a diffraction grating element to produce three beams. Of those three beams, two sub beams are directed to a predetermined pit on a recording track of the optical recording medium at predetermined angles with respect to a tangential direction to the track. The amounts of the light beams reflected by the optical recording medium are detected by a photo detector. Those light amounts detected are compared to generate a tracking error signal.
Accordingly, it is necessary to adjust the diffracting directions of the diffraction grating element with high precision so that the two sub beams hit the pit at the target angles with respect to the track tangential direction. In other words, it is necessary to accurately adjust the diffracting directions of the grating element by turning the grating face of the grating element about the optical axis. If the angular adjusting accuracy is poor, an accuracy of the tracking error signal is deteriorated.
To cope with this, the related technique adopts a mechanism resembling an optical-component mounting adjustment structure, which is disclosed in Japanese Utility Model Publication No. 63-30801B, for the position adjustment of the grating element. In an example of the related technique, a holder which rotatably holds a diffraction grating element is inserted into a cylindrical mounting hole formed in a frame body of the optical pickup device. An eccentric pin is inserted into a groove formed on the outer circumferential surface of the holder, and the eccentric pin is turned about an eccentric shaft oriented orthogonally to the outer circumferential face of the holder. With rotation of the eccentric pin, the holder rotates about the optical axis, so that the grating element may be adjusted to a target angular position.
However, the related grating-element angular position adjusting mechanism using the eccentric pin has the following problems to be solved.
The groove, which is formed in the outer circumferential face of the holder into which the eccentric pin is to be inserted, needs a length containing a rotation locus of the eccentric pin as viewed in the optical axis direction. If the length is short, the eccentric pin will run out of the groove. This fact makes it impossible to reduce the holder length, and provides a hindrance in reducing the device size by reducing its installation space.
By the rotation of the eccentric pin that has been inserted into the holder, force exerts on the holder in the rotational direction, and further it is pressed in the optical axis direction. Because of the pressing force, sometimes, the holder having inserted into the mounting hole of the frame body slightly inclines with respect to the axial line of the mounting hole.
If the holder, while being inclined, is fastened to the mounting hole by adhesive, the optical axis of the grating element which is held by the holder is inclined with respect to the optical axis of the laser light source. A laser beam is refracted by the inclined grating element, and as a result, a light emitting position is off its normal position.
Further, if the holder, while being inclined, is fastened to the frame body by adhesive, the holder is likely to shift from its normal position because of hardening shrinkage and temperature shrinkage of the adhesive, and the like. In other words, the grating element held by the holder is likely to shift from its correction position.
Additionally, when the grating element is turned by the eccentric pin, a relatively large force is required, and hence its fine adjustment is difficult.
Accordingly, the first object of the present invention is to provide a diffraction grating element with a position adjusting mechanism capable of reducing the length of the grating element and the mounting section, and to provide an optical pickup device with the same mechanism.
The second object of the present invention is to provide a diffraction grating element provided with a position adjusting mechanism which is free from such an undesired circumstance that during the position adjustment, undesired forces act on the grating element side, and as a result, the grating element is inclined with respect to the optical axis, and to provide an optical pickup device with the same mechanism.
The third object of the invention is to provide a diffraction grating element provided with a position adjusting mechanism capable of making a simple position adjustment, and to provide an optical pickup device with the same mechanism.
In order to achieve the above objects, according to the present invention, an arm portion is formed protruding outward from a holder of a diffraction grating element, and the arm portion is protruded outside a frame body, whereby the diffraction grating element is angularly positioned by operating the arm portion.
In the configuration, a length of the diffraction grating element as viewed in the optical axis direction can be reduced when comparing with the related art in which the rotation adjustment of the diffraction grating element is carried out by the eccentric pin. This feature is advantageous in reducing a size of a device having the diffraction grating element assembled therein. In the invention, there is hardly forces act on the diffraction grating element in other directions than the rotational direction as in the case using the eccentric pin. Therefore, the optical pickup device of the invention successfully avoids such a problem that the diffraction grating element is mounted in a state that it is inclined to the optical axis.
Further, the holder includes an outer circumferential face formed thereon, which is used as a reference for the optical axis alignment. It further includes an annular end face formed thereon, which is used for the positioning in the optical axis direction. With the structure, the optical axis alignment and the positioning in the optical axis direction are carried out by merely attaching the diffraction grating element to the mounting section of the frame body side. As a result, the mounting work of the grating element is simple.